The present invention relates to multilayer packaging films, for example, useful in applications requiring a high degree of dimensional stability at elevated oven temperatures, and more particularly to dual-ovenable, heat-sealable packaging films.
For the convenience of consumers, packaged food may be designed to be placed directly in a microwave or conventional oven to heat or cook the food without first removing the packaging. The consumer thus avoids having to handle the raw product or to clean a container in which the food would have otherwise been placed for cooking or heating. The consumer may also simply dispose of the packaging material after heating or cooking the food.
Packaging that can withstand exposure to the heating and/or cooking environment of a selected type of oven is said to be “ovenable” with respect to that type of oven. To be ovenable with respect to a microwave oven, the packaging should not, for example, include materials such as metals that reflect microwaves to cause arcing or otherwise damage the oven's microwave generation. To be ovenable with respect to a conventional oven, the packaging should, for example, be able in use to withstand exposure to 400° F. air temperature for up to four hours. Packaging that is ovenable both with respect to a microwave oven and a conventional oven is said to be “dual-ovenable.”
Food packaging may be formed by heat sealing thermoplastic film to itself to form a pouch or similar article containing the food. This heat sealing operation typically occurs at the food packager's plant using a heat sealing machine designed for high speed operation. Although there are several variations, generally a heat sealing machine includes a heated seal bar that contacts and compresses the two films to be heat sealed together. Three variables are important in forming a heat seal: 1) the seal bar temperature, 2) the dwell time, and 3) the sealing pressure. The seal bar temperature is the surface temperature of the seal bar. The dwell time is the length of time that the heated seal bar contacts the film to transfer heat from the seal bar to soften at least a portion of the films (e.g., the sealing layers of the films) so that they may be melded together. The sealing pressure is the amount of force that squeezes the films together during this heat transfer. All of these variables interact in completing a successful heat seal.
Because the heat sealing layers for much of the thermoplastic packaging films used in food packaging are based on relatively low-melting polyolefin thermoplastics (or similar melt-temperature thermoplastics), the heat sealing machines present in food packaging plants are often designed and set to operate with a seal bar temperature, a dwell time, and a sealing pressure in a range useful for such materials. This permits the heat sealing machines to operate at high speeds to form strong seals. Such a heat sealing machine may operate at, for example, a seal bar temperature of 290° F., a dwell time of 0.5 seconds, and a sealing pressure of 40 psig.
An existing ovenable packaging material for conventional ovens is a monolayer film based on a blend of nylon 6 with nylon 6,6. However, this film requires a relatively high sealing temperature to effect a useful heat seal. For example, at a sealing pressure of 40 psig and a dwell time of 0.5 seconds, the sealing bar temperature is generally at least about 380° F. Because typical existing heat sealing machines in food packaging plants cannot easily accommodate operation at those conditions, rather than forming a heat seal, a metal clip is typically used to close food-packaging based on this film, in which case the resulting food packaging is not dual ovenable. Further, a metal clip is an expensive closure method compared to heat sealing—and limits the use of X-ray examination to check packages for metallic contaminants.
The addition of a heat seal layer of relatively low-melting polyolefin thermoplastic to the existing monolayer film of a blend of nylon 6 and nylon-6,6 would result in a two-layer film that is heat sealable at the “polyolefin-type” of heat sealing conditions discussed above; however, such a film would not be able to form heat seals that are ovenable in conventional ovens because the relatively low-melting polyolefin layer would melt, decompose, or delaminate from the nylon layer upon exposure to a 400° F. conventional oven for three hours.